media: lirc: document BPF IR decoding

This is just a start.

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/media/uapi/rc/lirc-dev-intro.rst

@@ -20,6 +20,9 @@ data between userspace and kernelspace. Fundamentally, it is just a chardev
 file_operations defined on it. With respect to transporting raw IR and
 decoded scancodes to and fro, the essential fops are read, write and ioctl.
 
+It is also possible to attach a BPF program to a LIRC device for decoding
+raw IR into scancodes.
+
 Example dmesg output upon a driver registering w/LIRC:
 
 .. code-block:: none
@@ -34,6 +37,16 @@ What you should see for a chardev:
     $ ls -l /dev/lirc*
     crw-rw---- 1 root root 248, 0 Jul 2 22:20 /dev/lirc0
 
+Note that the package `v4l-utils <https://git.linuxtv.org/v4l-utils.git/>`_
+contains tools for working with LIRC devices:
+
+ - ir-ctl: can receive raw IR and transmit IR, as well as query LIRC
+   device features.
+
+ - ir-keytable: can load keymaps; allows you to set IR kernel protocols; load
+   BPF IR decoders and test IR decoding. Some BPF IR decoders are also
+   provided.
+
 .. _lirc_modes:
 
 **********
@@ -129,12 +142,28 @@ on the following table.
 
     This mode is used only for IR send.
 
+********************
+BPF based IR decoder
+********************
 
-**************************
-Remote Controller protocol
-**************************
+The kernel has support for decoding the most common IR protocols, but there
+are many protocols which are not supported. To support these, it is possible
+to load an BPF program which does the decoding. This can only be done on
+LIRC devices which support reading raw IR.
 
-An enum :c:type:`rc_proto` in the :ref:`lirc_header` lists all the
-supported IR protocols:
+First, using the `bpf(2)`_ syscall with the ``BPF_LOAD_PROG`` argument,
+program must be loaded of type ``BPF_PROG_TYPE_LIRC_MODE2``. Once attached
+to the LIRC device, this program will be called for each pulse, space or
+timeout event on the LIRC device. The context for the BPF program is a
+pointer to a unsigned int, which is a :ref:`LIRC_MODE_MODE2 <lirc-mode-mode2>`
+value. When the program has decoded the scancode, it can be submitted using
+the BPF functions ``bpf_rc_keydown()`` or ``bpf_rc_repeat()``. Mouse or pointer
+movements can be reported using ``bpf_rc_pointer_rel()``.
 
-.. kernel-doc:: include/uapi/linux/lirc.h
+Once you have the file descriptor for the ``BPF_PROG_TYPE_LIRC_MODE2`` BPF
+program, it can be attached to the LIRC device using the `bpf(2)`_ syscall.
+The target must be the file descriptor for the LIRC device, and the
+attach type must be ``BPF_LIRC_MODE2``. No more than 64 BPF programs can be
+attached to a single LIRC device at a time.
+
+.. _bpf(2): http://man7.org/linux/man-pages/man2/bpf.2.html